As a lubricant deterioration sensor, an oil deterioration sensor in which an oil intrusion clearance for intrusion of a lubricant is formed on an optical path from an infrared LED (Light Emitting Diode) to a photodiode has been known. The oil deterioration sensor detects an amount of light which exits from the infrared LED and is absorbed by the lubricant in the oil intrusion clearance based on an amount of light received by the photodiode, and determines a degree of deterioration of the lubricant that correlates to the detected amount of absorbed light (see; for instance, Patent Documents 1 and 2).
However, the oil deterioration sensor described in Patent Documents 1 and 2 can detect a concentration of insoluble substance in the lubricant as a degree of deterioration of the lubricant but has a problem that types of contaminants in the lubricant can not be specified.
As a technique for specifying a type of contaminant in a lubricant, a technique in which a light is irradiated to a membrane filter by an LED after filtration of a lubricant is known. In the technique, a light reflected from contaminants on the membrane filer is converted by a light receiving element into RGB digital values, and types of the contaminants in the lubricant are specified according to the converted RGB digital values (see; for instance, Non-Patent Documents 1 and 2).